Researchers found that the mammal can induce the condition when there is little food around and turn it off when food is abundant.

They believe it is a unique ability in the animal kingdom and results from the mammal’s need to maintain high blood sugar levels to feed its big brain.

But they also believe it is an ability that humans have lost through evolution and that studying dolphins could lead to techniques to re-activate it.

“Diabetes now accounts for five per cent of human deaths globally,” said Dr Stephanie Venn-Watson, a veterinary epidemiologist and director of Clinical Research at the National Marine Mammal Foundation in San Diego.

“It is our hope that this discovery can lead to novel ways to prevent, treat and maybe even cure diabetes in humans.”

More than 2.5 million people in Britain suffer from the condition, which can lead to serious complications including blindness.

Experts predict that up to four million Britons could be diabetic by 2025, in part because of the obesity crisis.

Dr Venn-Watson, who revealed her findings at the American Association for the Advancement of Science, made the discovery while researching dolphins off the San Diego coast.

By taking regular blood samples of the dolphins, she discovered that they could induce type II diabetes at times of fasting and then almost immediately turn it off again when food became available.

She believes that the ability dates back to when dolphins reverted from land animals to sea animals 55 million years ago and had to adapt to a protein-only fish diet.

She said there was evidence that humans may have done the same during the last ice age when they too had to rely on a protein rich diet because all carbohydrate rich foods had been frozen. This ability is now “dormant” she said but there is evidence that a fasting gene does still exist in humans.

“Maybe this is a smoking gun for a key point to control diabetes in humans,” she said.

“Maybe this is a vestige of something dormant that could be awakened and used as a therapy or cure.”

She believes that researching the dolphin’s DNA to work out how they do it could result in therapies in humans to switch on the ability again.

“Dolphins in the ocean go in to feast or famine situations”, she said. “They will eat a bunch of fish at once and then they may go a while and fast and not eat.

“During that fasting state they need a mechanism to keep sugar pumping around their blood.

“Dolphins can switch off diabetes but people cannot. Could there be gene therapies that control that switch if it exists in humans. If we could control that switch like dolphins it could be a cure.

“Then identifying and controlling such a switch could lead to possibly a cure for type II diabetes in humans.”

She said there was no desire to turn dolphins into laboratory animals and that many “volunteered” for research, approaching the beach and putting their tails in the air to have their blood taken.

But Danny Groves, spokesman for the Whale and Dolphin Conservation Society, said it opposes any research using dolphins.

“It is a grave concern that dolphins might be used in biomedical research,” he said.

“Dolphins are intelligent and sophisticated animals which are vulnerable to stress and suffering when confined and removed from their natural environment and societies.

“The fact that dolphins in captivity experience ongoing stress adds to questions about the validity of studies of physiological processes that are intimately connected with the animals' well-being.”